Mitochondrial DNA (mtDNA) molecules from different species of the genus Drosophila differ in size from 15.7 to 19.5 kilobase pairs (kb). These differences can be accounted for by differences in size (1.0 to 5.1 kb) of a single adenine and thymine (A plus T) -rich region in each molecule. The A plus T-rich regions of mtDNA molecules of a number of different species (D. melanogaster, D. simulans, D. mauritiana, D. yakuba, D. takahashii, and D. virilis) has been shown to be located at homologous positions in the molecules, and to contain the molecule's single origin of replication; however, they differ extensively in regard to their nucleotide sequences. In order to gain information on the differences in nucleotide sequences in A plus T-rich regions of mtDNA molecules of a number of different Drosophila species, we are attempting to sequence specific segments of A plus T-rich regions from cloned or natural mtDNAs. Fusion experiments will be carried out using tissue culture cells from D. melanogaster and other Drosophila species whose mtDNA molecules differ from those of D. melanogaster in regard to restriction site locations, and size of A plus T-rich regions. Attempts will be made to identify recombinant molecules in mtDNA from hybrid cells in order to learn about the mechanism by which exchanges between molecules are made.